Discharge tube



Patented Jan. 19, 1937 PATENT OFFICE DISCHARGE TUBE Donald v. Edwards, Montclair, and Earl K. Smith, East Orange, N. J., assignors toElectrons, Inc. of Delaware, a corporation of Delaware Application November 4, 1932, Serial No. 641,152 Renewed April 6, 1936 S Claims.

This invention relates to gaseous discharge tubes and has particular reference to the type of such tubes known as control rectifiers wherein the starting of an arc discharge between the cathode and anode is controlled by a third element.

The object of the invention is to provide a tube of this type in which the trigger voltage, 1. e., the control or grid voltage at which the discharge starts, will be substantially independent of the filament activity throughout the life of the tube.

A further object of the invention is to reduce the energy required tocontrol a tube of this type and to make the value of such energy independent of filament activity.

The invention will be described with reference to the embodiment thereof shown in the accompanying drawing, in which Fig. 1 is a diagrammatic view partly in section illustrating a tube embodying the invention and circuits adapted to make practical use thereof;

Figs. 2 and 3 are curves illustrative of certain features of the invention;

Fig. 4 is a sectional view of a modified form of cathode: V

Fig. 5 is a plan view thereof.

Referring to Fig. 1, i represents an envelop containing a gaseous filling,preferably argon, neon or other inert gas. The cathode comprises a heat shielding can or enclosure 4 which may be substantially closed as shown in Fig. 1, or open ended and sectional as shown in Figs. 4 and 5. In either case the upper portion of the can is open ended, either by the provision of an opening 5 of limited area as shown in Fig. 1, or a predetermined diameter of the shield as shown in Figs. 4 and- 5. The area of opening 5 will determine the cross sectional area of the electron stream before starting and the manner in which this area is determined will be hereinafter described.

The cathode also contains one or more heatable filaments 3, 3. These may be arranged by having a plurality of such filaments attached to the upper flange of the can and the lower ends thereof connected to a connecting plate 25 as shown in Fig. 1, or one or more elements connected across the respective sections of the shield as shown in Figs. 4 and 5. r

In Fig. 1 the plate 25 is supported by a collar 26 of refractory insulating material 26 which surrounds the supporting pin 21 of conducting material, forming a heating circuit from the transformer coil l5, conductors 9, cathode supports 23 and shielding can 4 to one end of the filaments 3, thence by conducting pin 2! to the return lead 8. The cathode may be of suitable emissive material, either coated with alkaline earth oxides or of suitable metallic type such as nickel or barium.

In the form of cathode shown in Fig. 4 the circuit will be from the transformer I5 by way of one cathode support to one of the segments of shield 4, thence by filament 3 to the opposite shield and return support and conductor. The shields are insulated from each other by suitable separation. The filament is supplied preferably with alternating current from the alternating current source I4 supplying current to the primary of the transformer I2.

The anode 1 may be in any suitable form. Preferably it is shown in the form of a plate I supported from the upper press by conducting supports connected to the anode lead H which leads byway of a load resistance and switch 2| to the main generator l9, one pole of which is connected to the middle point of the secondary winding 15. Likewise, the control element or grid li may be of any suitable construction. A

' convenient form comprises a circular flanged plate 6 having a flanged rim 3| and a central opening 32 across which are one or more bars or conductors 33. The grid structure is preferably supported to the upper press by means of supports 34, 34 supported from the stem, and by an auxiliary support 35 fixed to the cross brace 36, the latter being attached to the supports 34, thereby supporting the grid from two separated points on the upper press. The support 35 is conducting and connected to the grid lead 31.

To start the discharge in a control rectifier of this type electrons must first be emitted from the filament and pass into the space'between the grid and anode. For a given grid plate configuration, gas filling and plate voltage, the value of the electron current necessary to start a discharge will vary with the grid voltage and will be the same for all tubes of a given structure provided the electrode surfaces are clean and other conditions remain constant. The electron current which will flow from a given filament through a given shield opening varies considerably between tubes of the same structure and also varies considerably in a given tube throughout its life. The designer may control it approximately by arranging the filament surface near to the grid and providing a large opening to the grid in the shield opening. We have found that if the grid filament configuration has been arranged to give a larger electron current before discharge than a certain critical value, the grid voltage necessary to initiate a discharge becomes constant, whereas if it is less than the critical value the voltage is highly erratic and changes with filament activity in an unpredictable and uncontrollable manner.

This relation is shown in Fig. 3 in which we have plotted the results of ,dneasurements upon a series of. tubes of similar constructions. The ordinates represent the grid potential necessary to start the tube, sometimes referred to as the the trigger voltage. The abscissae represent the area of the opening of the electron stream through the shield. From this diagram it will be noted that as the size of the opening increases the necessary trigger voltage decreases until a critical point is reached, after which it becomes substantially constant.

If the opening is increased still further it will be found that at a certain large opening the current flowing in the grid circuit prior to the discharge will reverse its direction; that is to say, instead of there being an excess of ions arriving at the grid, it will nowlbe found that an excess of electrons arrive. e magnitude of this electronic grid current varies erratically with filament activity, whereas the ion current was independent of filament conditions, being relatively small and varying mainly with grid surface conditions. This latter condition is shown in Fig. 2, in which we have plotted the results of a series of tests and in which the ordinates represent the grid current flowing immediately prior to the time the discharge starts, and the abscissae represent the area of the shield opening. To control a tube having this electronic grid current requires that suffioient energy shall be present in the grid circuit to overcome the voltage developed in the various grid impedances by the said current. Hence the electronic grid current tends to cause the operation of the tube to become highly erratic. The value of this current may be several hundred times that of the ionic current, and hence the energy thereof is largely increased.

It will be noted from the curve of Fig. 2 that as the area of the shield opening increases the grid current remains a substantially constant ionic current until a critical valve 17 is reached, at which time the current becomes electronic and rises rapidly.

By designing the opening in the shield to provide a cross sectional area of electron stream to lie between these two extremes, i. e., between the minimum opening where the trigger voltage becomes erratic due to insufiicient electron current prior to discharge, and the maximum where the grid current becomes electronic and also erratic, we have been able to build tubes, the trigger or starting voltage of which in a normal circuit is independent of the filament activity throughout the life of. the tube.

In practice, in applying these principles we proceed as follows: From the discharge current rating and control ratio desired the size of the cathode, plate and grid are designed. From the operating voltages and conditions a suitable gas or vapor filling is chosen and the grid and plate configuration determined. A trial tube is then built in which the filament configuration and the opening in the shield are believed to be so large that the tube is sure to have a reverse grid current. The tube is then operated as a control rectifier, as shown in Fig. 1, at a very low load, at the rated plate voltage and at gradually reduced filament voltages, measurements of the grid current immediately before discharge being taken in the usual manner by comparison of the trigger points with and without a known resistor 36 in the grid circuit, which latter is connected by contact 24; to a potentiometer 23 whereby a battery 22 may supply positive or negative potential to the grid, the grid lead being connected from the potentiometer to the midpoint of the transformer secondary l5. At the point where the grid current becomes ionic the filament voltage is noted. These measurements may be made by the voltmeters V and V1 connected as shown. The filament temperature is again lowered until the point is reached where no discharge or flickering occurs at the normal trigger point and it becomes necessary to make the grid potential more positive to obtain flickering. The filament voltage is again noted at this point. The absolute value of the electron current from the filament before the discharge is not readily measurable, but a relative value may be obtained for design purposes by disconnecting the plate and measuringthe electron current to the grid with a micro-ammeter 38 at some voltage below ionization potential of the gas, say at about plus 4 volts. The said relative value is thus measured at each of the above noted filament voltages and also at normal filament voltage, preferably under conditions which give the range of activity for the particular type of filament coating or filament emissive material used. In general this should be after operating 48 hours with filament lighted but no load, and also instantly after carrying full load.

From the above data a final design is made so that the relative electron current at normal filament voltage and temperature will always be under the maximum corresponding to electronic grid current and above the minimum corresponding to failure to start at the true trigger point. With simple shielding designs the electron stream may be considered to vary approximately in proportion to the area of the opening.

It is possible to choose a grid structure such that the minimum opening through the shield may be larger than the maximum as above measured. In this event the grid must be redesigned to present less surface directly to the filament and to increase the concentration of the discharge through the grid. We prefer a design such that the maximum and minimum are fairly far apart in order to allow for the normal variation of emissivity. There are limits which prevent carrying this too far. For instance, if the grid surface presented to the filament is reduced too much the tube becomes susceptible to stray fields and wall charges, or, if the current through the grid is concentrated too much, difiiculty will be encountered in radiating the heat from the grid or in getting a sufiiciently high peak forward. voltage. If the grid is placed too close to the filament diificultly is encountered from sputtered active material. However, the maximum and minimum separation need be only enough to allow for the operating range of activity and a slight factor of safety.

Precautions should be taken to insure that the grid and the plate, especially the former, are well cleaned of electropositive materials, and also that there is no slight glow discharge between the parts of the support members of the grid and plate at operating voltages.

Greater accuracy may be obtained if the phase of generator M is shifted with respect to I9 so that the instantaneous filament voltage is zero at the time the discharge starts. In this way the trigger voltage indicated by voltmeter V1 is the true trigger voltage and does not include any voltage drop across the filament.

In the construction shown in Fig. 1 the filaments 3 are electrically connected at one end to the shield 4 and at the other ends to the common member 25 which is insulated from the shield. This improves the shielding action by making the shield positive to one end of the filaments for a portion of each cycle of heating current supplied from the winding l5, thereby tending to maintain filament activity during long periods when the filament may be lighted without load upon the tube.

In the construction shown in Fig. 4 each half of the shield is positive to the other half and to the other end of the filament during each cycle of the heating current, and thus maintains filament activity during periods without load.

The control tubes above described will start at the designed grid potential under all starting conditions throughout the life of the tube. They may also be manufactured with much greater uniformity because in this way the tube characteristics are independent of the filament activity which cannot easily be controlled within small tolerances.

Having described our invention we claim:-

1. In combination, a cathode adapted to be rendered electron emissive by an electric current passing therethrough and a shield around said cathode, said shield being divided into two parts insulated from each other and connected to the respective ends of the cathode 2. In a gaseous discharge device including an anode, a cathode, and a control grid electrode, a grid-cathode structure wherein said grid and said cathode are mounted in predetermined operative relation and said cathode is provided with a shield having an opening therein between said grid and said cathode; the said grid-cathode structure, including the dimensions of the opening in said shield, being such that, with normal excitation of said cathode and less than discharge voltage on said grid, the current through said grid is ionic.

3. In a gaseous discharge device including an anode, a cathode, and a control grid electrode, a grid-cathode structure wherein said grid and said cathode are mounted in predetermined operative relation and said cathode is provided with a shield having an opening therein between said grid and said cathode; the said grid-cathode structure, including the dimensions of the opening in said shield, being such that, with normal excitation of said cathode, the grid current immediately before a gaseous discharge is approximately zero, and the electron stream from said cathode is above the minimum necessary to start a discharge upon the application of discharge or "trigger potential to said grid.

4. In a gaseous discharge device including an anode, a cathode, and a control grid electrode, a grid-cathode structure wherein said grid and said cathode are mounted in predetermined operative relation and said cathode is provided with a shield having an opening therein between said grid and said cathode; the said grid-cathode structure including the dimensions of the opening in said shield, being such that, with normal excitation of said cathode, the grid current immediately before a gaseous discharge is approximately zero, the electronic and the ionic componets in said grid current being balanced.

5. In a gaseous discharge device including an anode, a cathode, and a control grid electrode, a shield for said cathode having an opening therein for determining the exposure of said grid to said cathode, said grid being disposed in predetermined operative relation with respect to said cathode in such manner that with normal excitation of said cathode the size of said opening effectively controls the electron emission from said cathode with respect to said grid and is within the limits of a maximum opening below which the grid current is maintained ionic prior to discharge, and a minimum opening above which discharge is efi'ected uniformly at the same trigger voltage applied to the said grid.

6. In a gaseous discharge device including anode, cathode, and control electrodes, a cathodecontrol electrode structure wherein said control electrode and said cathode are mounted in predetermined operative relation with regulating means disposed therebetween, and so constructed and arranged that with normal excitation of said cathode the effective electron emission from said cathode is suflicient to start a discharge upon the application of discharge potential to said control electrode, while the current through said control electrode immediately prior to said discharge is substantially zero, whereby the value of the discharge potential is maintained uniform and free of the variable influence of an erratic current in the cathode-control electrode prior to the start of discharge.

7. In combination, a cathode adapted to be rendered electron emissive by an electric current passing therethrough, and a shield around said cathode, said shield comprising a pair of substantially semi-cylindrical portions spaced from each other and connected to the respective ends of the cathode.

8. In combination, a cathode adapted to be rendered electron emissive by an electric current passing therethrough, a shield around said cathode, said shield comprising a pair of substantially semi-cylindrical members open at opposite ends with their longitudinally extending edge portions spaced from each other, and connections between opposite ends of said cathode and the respective semi-cylindrical portions of said shield.

. DONALD V. EDWARDS.

EARL K. SMITH. 

